In recent years, development of an electron device (compound semiconductor device) in which a GaN layer and an AlGaN layer are sequentially formed over a substrate, and the GaN layer is used as an electron transit layer is performed vigorously. A GaN based high electron mobility transistor (HEMT) is cited as one of the compound semiconductor devices as stated above. In the GaN based HEMT, high-concentration two-dimensional electron gas (2DEG) generated at a heterojunction interface between AlGaN and GaN is used.
A band gap of GaN is 3.4 eV, which is larger than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV). Namely, GaN has high breakdown electric field intensity. Besides, GaN also has large saturation electron velocity. Accordingly, GaN is very expectable as a material of a compound semiconductor device capable of high-voltage operation and high-power. The GaN based HEMT is expected as a high breakdown-voltage electronic device used for a high-efficiency switching element, an electric vehicle, and so on.
The GaN based HEMT using the high-concentration two-dimensional electron gas performs a normally-on operation in many cases. Namely, a current flows when a gate voltage is turned off. This is because a number of electrons exist at a channel. On the other hand, a normally-off operation is regarded as important from a point of view of fail-safe for the GaN based HEMT used for the high breakdown-voltage electronic device.
Accordingly, various investigations have been done as for the GaN based HEMT capable of the normally-off operation. For example, a structure in which a p-type semiconductor layer is provided between a gate electrode and an active region is proposed. Besides, a structure dividing the 2DEG by etching an electron supply layer just below the gate electrode is also proposed.
However, doping of p-type impurities and a heat treatment for activation are necessary to obtain the structure providing the p-type semiconductor layer. It is necessary to increase a temperature of the heat treatment to high temperature because the p-type impurities are extremely difficult to be activated compared to n-type impurities, and mobility of electrons is lowered because an interface between the electron transit layer and the electron supply layer is damaged during the high-temperature heat treatment. Besides, significant damage occurs in a vicinity of the electron transit layer in the etching to obtain the structure in which the 2DEG is divided, and therefore, there are cases when a sheet resistance increases and leak current increases. Accordingly, it is difficult to apply these technologies to actual devices.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2007-19309
Patent Literature 2: Japanese Laid-open Patent Publication No. 2009-76845